We got a water taxi from Reduit beach which was 80EC return for two. The boat driver was great and it is only 10 minutes across the bay.
When booking we were advised to allow about 3 hours which was about right.
The boat dropped us at the little dock where one of the park staff showed us where to go to pay the admission which was 8USD each. The whole island and the ruins of the Garrison buildings are interesting, the walk up to the Fort where the cannons are is not that strenuous, they advise shoes but we found it easy enough if flip flops as long as you take your time. I would advise taking water with you.
The views of Rodney Bay from the Fort are fantastic as are they from Signal Hill and you can also see Martinique from there (which is why the Fort was actually built there).
There is a small restaurant next to the dock, we had a couple of drinks there while we waited for the boat. The drinks were cheap and although we did not eat we saw some that did and the food looked very good and again not expensive.
All in all this little trip was well worth doing and I got to learn a little more history.More